Sound Navigation and Ranging (SONAR) equipment transmit a sonic or ultrasonic pulse. The pulse is reflected from an object, and received back at the transmitter location. The elapsed time for the pulse gives target distance, and the directional characteristics of the transmitting-receiving transducer give target range. For underwater communication the pulses may be spaced to provide a communication code such as the Morse code, for example.
A problem with these original type sonar devices is that the pulses are readily detectable by listening devices located on the target or other stations. From this information the target or other stations can determine the direction and range to the transmitting sonar or decode the communication or both.
To maintain the secrecy of the sonar transmitter and its communications various sonar systems have been developed. For example, it has been known to tape the natural ambient noises at a location in a body of water whose depth is to be measured. The recorded noise signals are projected back into the surrounding water to reflect off the bottom. The return signals are received for correlation with the projected recorded noises. The time delay required to correlate the signals is used to determine the depth of the water. Those persons desiring more information for this system are referred to U.S. Pat. No. 4,086,560 issued Apr. 25, 1978 to Johnston et al for a Secret Depth Sounder.
Another known device modulates transmitted intelligence signals with one or more disturbing oscillatory signals within a whole frequency band, such as that of the human voice, while maintaining the amplitude thereof only slightly higher than the maximum amplitude of transmitted intelligence signals. The signals of this system are easily detected and any code decoded. To make such system secure, an encoded echo-ranging signal generator was developed which included a digital shift register having binary encoded stages. A digital message encoder has a corresponding number of stages connected to the stages of the digital shift register for setting a predetermined program of zeros and ones in accordance with a selected code.
A shift pulse generator is provided to timely shift the shift register. An anti-coincidence circuit has a pair of inputs connected, respectively, to a predetermined stage and output of the digital shift register. The output of the circuit is connected to the input of the digital shift register. Thus, the binary contents of the encoded stages are randomly recirculated through the shift register and out through an echo-ranging system. This device overcomes substantially the disadvantages of the preceding devices, as the encoding of the transmitted intelligence signal may be made so complex that it is practically impossible to decode it without having appropriate decoding equipment having the key thereto. The device is advantageous in the sonar, radar, or other echo-ranging fields. Those persons desiring additional information for the system are referred to U.S. Pat. No. 3,958,214 issued May 18, 1976 to Andrews, Jr. et al for an Encoded Echo-Ranging Signal Generator.
A disadvantage of the known systems is their failure to make it virtually impossible for a target to discover that it is under surveillance and messages are being transmitted in their vicinity.